Why does my heater in my 2000 ford ranger only work on the high setting?

I assume that you are talking about the heater fan?

Your heater FAN has at least 3 settings, and for our purposes we will assume low,med and high

Low and medium run through a resistor High is wired direct
Either your resistor is bad or your fan is
I know it seems wrong that the fan could be shot but it could be pulling so many amps that it won't work in any other mode than high
I just had this happen on one of my trucks
Start with the fan